Testbeam studies of irradiated modules for the ATLAS ITk Strip upgrade

To cope with the occupancy and radiation environment expected at the High-Luminosity LHC, the ATLAS experiment will replace its Inner Detector with an all-silicon Inner Tracker (ITk), containing pixel and strip subsystems. The strip subsystem will be built from modules, consisting of one or two n+-in-p silicon sensor(s), one or two PCB hybrid(s) containing the front-end read-out electronics, and one powerboard with high voltage, low voltage, and monitoring electronics. The sensors in the central barrel region of the detector will use a simple rectangular geometry, while those in the forward end-cap regions will use a radial geometry with a built-in stereo angle. To validate the expected performance of the ITk strip detector, a series of testbeam campaigns has been performed over several years at the DESY-II testbeam facility. Tracking was provided by EUDET telescopes, consisting of six Mimosa26 pixel planes. An additional FE-I4 pixel plane was used to provide sufficient timing resolution for the telescope. In the years 2021-2022, the focus of testbeam campaigns has been on assessing module performance post-irradiation, using the final production versions of the sensors, and most recent versions of front-end electronics. Three modules of differing geometry were built from irradiated components; a barrel Short Strip (SS), an end-cap R0, and an end-cap R5 - the first "split" module (containing two sensors) to be tested at testbeam. Measurements of the collected charge, detection efficiency, and noise occupancy were performed on all tested modules, as well as of the tracking performance in various sensor regions. The results give confidence in the operability of the detector across its lifetime.